Add check-only formatting mode with diff and concise output

[coderabbitai]

Summary

Add a --check mode that detects files which mdtablefix would change without modifying them.

In normal --check output, emit a diff for each changed input file.

Add a --concise flag for --check mode. This flag must list only the changed filenames.

Rationale

Repositories need a check-only formatting gate that reports formatting drift without changing tracked files.

A diff enables users to diagnose the required formatting changes. Filename-only output enables concise CI logs.

Required changes

• Add a --check command-line option.
• In --check mode, process every input file without modifying any input file.
• Return exit status zero when no input file requires formatting changes.
• Return a non-zero exit status when any input file requires formatting changes.
• In normal --check mode, emit a readable diff for every input file that requires changes.
• Add a --concise command-line option.
• When --check --concise is used, list only the filenames of input files that require changes.
• Define the interaction of --concise without --check, including either rejection or a documented behaviour.
• Preserve existing formatting and in-place formatting behaviour outside --check mode.

Affected areas

• Command-line argument parsing.
• Formatter execution and file-writing paths.
• Diff rendering.
• User documentation and command help.
• Automated tests.

Acceptance criteria

• mdtablefix --check clean.md returns exit status zero and does not modify clean.md.
• mdtablefix --check unformatted.md returns a non-zero exit status and does not modify unformatted.md.
• mdtablefix --check unformatted.md emits a diff that identifies the required changes.
• mdtablefix --check processes all supplied input files and reports every changed file before it exits non-zero.
• mdtablefix --check --concise returns a non-zero exit status for changed input and lists only changed filenames.
• --concise output contains no diff content.
• Tests cover clean files, changed files, multiple input files, no source-file modification, exit statuses, diff output, and concise output.
• Command help documents --check and --concise.

Backlinks

• Originating pull request: Enforce Markdown formatting in check-fmt netsuke#619
• Originating comment: Enforce Markdown formatting in check-fmt netsuke#619 (comment)
• Requested by: @leynos

[leynos]

Superseded by the --check and --diff modes, which landed on the
check-option branch under ADR 0009. Closing as not planned, because the
--concise flag this issue proposed will not be added.

The acceptance criteria asked for --check to emit a diff and for a
--concise flag to narrow it. That split is implemented, but as two modes
rather than one mode plus a modifier:

• --check prints one line per drifting file, <path> +<inserted> -<deleted>.
  It is already the quiet form a CI log wants, so no second flag is needed to
  get it.
• --diff prints a unified diff per drifting file.

The reason --concise existed was to ask for both renderings from one mode.
With two modes, neither caller has to pass a flag it does not want, and the
two cannot be requested in contradiction of each other. Both modes are
read-only, both exit 1 when a file would change and 2 on an operational
failure, and both share one assessment with --in-place, so a gate cannot
disagree with the writer it is gating.

• Decision record: docs/adrs/0009-check-and-diff-reporting.md (Option A, with
  the rejected alternatives, including implementing this issue literally).
• Plan: docs/execplans/check-option.md, which carries the full record.
• User-facing reference: docs/users-guide.md, section "Command-line usage".